Power supply with a direct converter

ABSTRACT

A power supply includes a direct converter provided in the form of a two-phase or three-phase bridge circuit. The bridge branche of the direct converter includes a serial connection of any number of identical two-terminal networks, each having the following characteristics: The two-terminal networks each have at least one switching state, in which their terminal voltage takes on positive values independent of the magnitude and polarity of the terminal current; the two-terminal networks each have at least one switching state, in which their terminal voltage takes on negative values independent of the magnitude and polarity of the terminal current; the two-terminal networks have at least one internal energy store.

This application is the national phase under 35 U.S.C. § 371 of PCTInternational Application No. PCT/DE03/01286 which has an Internationalfiling date of Apr. 16, 2003, which designated the United States ofAmerica and which claims priority on German Patent Application number DE102 17 889.5 filed Apr. 22, 2002, the entire contents of which arehereby incorporated herein by reference.

FIELD OF THE INVENTION

The invention generally relates to a power supply with a directconverter which is in the form of a two-phase or three-phase bridgecircuit. A system such as this may be used in particular for supplyingpower to rail vehicles, but may also be used just as well for supplyingpower to stationary devices, for example in conjunction with amedium-frequency transformer.

BACKGROUND OF THE INVENTION

Recently, for rail vehicles using an AC overhead wire, increasingattempts have been made to find technical solutions to avoid the needfor the conventional network transformer. In particular, networktransformers have the disadvantages that they are heavy and they incurrelatively high energy losses.

One solution is to use superconducting transformers although, interalia, they are more problematic for vehicle use owing to the coolingsystems that are required, than for stationary systems, and aretherefore not yet technically proven at the moment.

Other known solutions require the direct connection of thepower-electronic converters to the high voltage of the AC overhead wire.Until now, the disadvantages here have been the high technicalcomplexity and a range of restrictions which prevent universal use, aswill be explained in the following text.

In general, modern electric locomotives use power-electronic convertersfor feeding the traction motors and the auxiliary systems(air-conditioning system and other modes). The overhead wire voltageswhich have been introduced throughout the world for AC locomotives have,however, been chosen to be very high. This is done in order to minimizethe transmission losses (in Europe: 15 KV/16 ⅔ Hz, as well as 25 KV/50Hz). While avoiding the need for a network transformer by beingconnected directly to the overhead wire voltage, these high voltagesrequire semiconductors or converter elements to be connected in seriesin the power electronics—as well as a high degree of technicalcomplexity overall.

The following variants with direct connection of the power-electronicconverters to the overhead wire voltage are known:

-   a) Use of high-voltage insulated traction motors without DC    isolation between the motors of the overhead wire, for example, is    known from Steiner et al, A New Transformerless Topology for AC-Fed    Traction Vehicles using Multi-Star Induction Motors, EPE 1999,    Lausanne. Solutions such as these have the advantage of a small    number of power-electronic converter stages in the energy flowpath    between the overhead wire and traction motors. This reduces the    energy losses and the complexity of the power electronics. One    example is shown in FIG. 6. On the other hand, the disadvantages    include:-   The increased motor isolation that is required has a disadvantageous    effect on the physical motor size, on the motor weight and/or on the    motor efficiency.-   The power matching that is absolutely essential between the    series-connected converter elements (when the motors are subject to    different loads) requires the use of complex motor windings with a    large number of connections carrying high voltages (so-called 3-star    motors).-   The number of traction motors cannot be varied freely. In order to    limit the disadvantageous effects of the items mentioned above, as    large a number of high-power motors as possible should be chosen.-   The high DC voltage components between the series-connected    converter elements make it harder to provide reliable isolation in    actual conditions (dirt, moisture).-   The feed for the auxiliary systems requires considerable additional    complexity.-   In order to maintain a restricted operating capability in the event    of failures in the power section or in the event of insulation    faults in the motors, additional switching devices are required    (redundancy).-   b) Use of conventional traction motors (with a low load on the    insulation) without DC isolation from the overhead wire close to    ground potential, for example as known from DE 197 21 450 C1. The    disadvantages and restrictions of the first-mentioned solution are    avoided with this embodiment of the motors. Further advantages are    good efficiency and low harmonic current levels in the railroad    network. However, the voltage transformation ratio required from the    high input voltage (up to 25 kV rated value) to the normal output    voltages for feeding the traction motors is about 10:1. In    principle, power-electronic converters which provide a high voltage    transformation ratio without the assistance of transformers are    worse, in terms of the complexity for energy stores and    semiconductor switches, than converters of the same rating that have    to provide only a low voltage transformation ratio. This fundamental    disadvantage, in particular the size of the energy stores that are    required, prevents universal use of corresponding variants.-   c) DC isolation of the motors from the overhead wire using a number    of individual medium-frequency transformers, which each have    associated individual, series-connected converter elements, for    example as known from Schibli/Rufer, Single and Three-Phase    Multilevel Converters for Traction Systems 50 Hz/16 ⅔ Hz, LEI,    Lausanne, pages 4.210-4.215. Variants of this solution have already    been very widely investigated. FIG. 7 shows a corresponding circuit    arrangement. The following items are characteristic:-   On the network side, groups of converter elements are connected in    series and, overall, produce a staircase voltage whose controllable    maximum values must be greater than the network voltage peak values.-   Each of these converter element groups has a four-quadrant    controller on the network side, a first DC voltage capacitor, a    medium-frequency inverter on the primary side, a medium-frequency    transformer, a medium-frequency rectifier on the secondary side, and    a second DC voltage capacitor. All of the converters must be    designed for both energy flow directions (energy drawn from the    railroad network as well as energy feedback), if it is intended to    be possible to feed energy back into the railroad network.-   A solution such as this is not subject to the disadvantages of the    first solution that was described (high-voltage insulated traction    motors). The insulation load on the traction motors can be kept low.    There are no restrictions to the number or operating voltages of the    traction motors. Power matching between motors that are subject to    different loads can be carried out via the DC busbar (P₀, N₀ in FIG.    7). On the other hand, the disadvantages are as follows:-   The large number of power-electronic converter stages which are    located in the energy flowpath between the overhead wire and the    traction motors (high degree of complexity, relatively high energy    losses).-   The high degree of complexity for energy stores (2 DC voltage    capacitors plus any series resonant circuits) for smoothing the    power pulsation at twice the network frequency.-   The large number of individual medium-frequency transformers    required, which in total are worse than one central transformer in    terms of the weight and the space required. Splitting into a large    number of individual transformers is also disadvantageous, and    occupies a large amount of space, as a result of the (in total)    numerous transformer connection points on the high-voltage side.-   As in the first-mentioned variant, the high DC voltage components    between the series-connected converter elements make it harder to    provide reliable isolation in actual conditions (dirt, moisture).-   The feed for auxiliary systems requires considerable additional    complexity.-   In order to maintain a restricted operating capability in the event    of failures in the power section or insulation faults in the motors,    additional switching devices are required (redundancy).-   d) DC isolation of the motors from the overhead wire using a    medium-frequency transformer which is fed from the overhead wire by    means of a direct converter, for example as known from DE 26 14 445    C2 or Östlund, Influence of the control Principle on a High-Voltage    Inverter System for Reduction of Traction-Transformer Weight, EPE,    Aachen 1989.

Solutions such as these likewise have the advantage of a reduced numberof power-electronic converter stages located in the energy flowpathbetween the overhead wire and the traction motors. If themedium-frequency is sufficiently high (in the order of magnitude ofabout 1 KHz or more), the physical size, weight and energy losses in themedium-frequency transformer may be kept considerably lower than thecorresponding disadvantages in the motors. In addition, it is possibleto feed the auxiliary systems from the medium-frequency transformerefficiently and with little complexity. In general, this isadvantageously done by means of a separate secondary winding on themedium-frequency transformer.

However, for several reasons, the provision of a direct converter withthyristors does not satisfy present or future requirements. The majordisadvantages are:

-   The achievable medium frequency is restricted to a few 100 Hertz    owing to the commutation times in a circuit fitted with thyristors.    This frequency is not sufficient to significantly reduce the weight    of the medium-frequency transformer.-   The stringent interference current limit values (minimizing harmonic    currents in the railroad network) for modern locomotives cannot be    complied with because, in principle, the spectrum includes twice the    medium frequency and other interference frequencies. Twice the    medium frequency is, furthermore, far too low to be adequately    damped by the inductive network impedance.-   The primary winding in the medium-frequency transformer is loaded    with the high peak voltage values of the overhead line voltage plus    their transient overvoltage spikes. This makes it harder to provide    isolation (winding isolation, air gaps, creepage paths) for the    transformer.

Direct converters with power semiconductors which can be turned off (ingeneral: IGBT transistors instead of thyristors) are known in the formof so-called matrix converters, for example from Kjaer et al, APrimary-Switched Line-Side Converter Using Zero-voltage Switching, IEEETransactions on Industry Applications, Vol. 37, No. 6, pages 1824-1831.FIG. 8 shows the basic circuit of a matrix converter with the converterbranches and the filter capacitor. The converter branches are providedin a known manner by way of bi-directional controllable electronicswitches. Known implementations are:

-   Two thyristors (GTO thyristors) which can be switched off and are    connected back-to-back in parallel. These components must have a    reverse blocking capability, that is to say they must be able to    block both voltage polarities (FIG. 9).-   Two IGBT transistors which are connected back-to-back in parallel.    These components must have a reverse blocking capability (FIG. 10).-   Two IGBT transistors which are connected to back-to-back    parallel-connected diodes. There is no need for components with a    reverse blocking capability (FIG. 11).

However, the following disadvantages of direct converters are alsoassociated with these embodiments. These are:

-   The low-frequency power pulsation (at twice the network frequency:    2f_(N)) which occurs in single-phase AC networks and must be    transmitted by the medium-frequency transformer. This has a    disadvantageous influence on the physical size and efficiency of the    medium-frequency transformer.-   The filter capacitor which is required on the network side and can    cause interference resonances in the railroad network, and which can    lead to the circuit having undesirably low input impedances for    higher-frequency interference currents.-   In contrast to converters with a DC voltage intermediate circuit (“U    converters”), the power semiconductors have no protection against    high-energy network overvoltages, as provided by a capacitor on the    DC voltage side. This necessitates comparatively considerable    derating of the semiconductor reverse voltages.-   The harmonic content of the converter voltages which are produced is    very high both on the network side and on the transformer side. No    suitable circuits or methods are known for producing staircase    voltages with a low harmonic content (analogously to multipoint U    converters) for matrix converters.

In addition, for the present matrix converter applications, it is ofmajor importance to be able to cope with high voltages and possiblemalfunctions without serious consequential damage in the relatively highpower range. Disadvantageous items relating to this are:

-   In the event of a short circuit on the AC voltage side between the    circuit points N₁ and N₂ (see FIG. 10), extremely high discharge    currents flow from the filter capacitor on the AC voltage side,    which can cause destruction, owing to the extremely high mechanical    forces and/or arc damage that occur.-   In the event of failure of power semiconductors or a faulty drive,    the discharge current, which is like a short circuit, can flow    directly through the semiconductors, destroying them and their    contacts.-   The very small stray inductance from the filter capacitor and from    the converter branches which is required for the semiconductor    switches in the matrix converter conflicts increasingly with the    rising voltage level (with a peak value of up to about 50 KV for a    25 KV overhead wire voltage) for a design embodiment which is    mechanically resistant to short circuits and is safe in terms of    isolation. Furthermore, there are major impediments to unrestricted    spatial arrangement of the components.

An arrangement as shown in FIG. 11 is known, inter alia from Kjaer, loc.cit. (see FIG. 3 there). In comparison to FIG. 8, this includes thefollowing three modifications:

-   The filter capacitor is split into a number of capacitors. However,    the resultant capacitance is still connected in parallel with the    network-side connections of the matrix converter, so that the    disadvantages also still remain.-   Additional damping resistors are connected in a known manner in    series with the filter capacitors. This measure results in high    energy losses, although it has become necessary because the filter    capacitors are additionally required as snubbers for the IGBTs.-   The medium-frequency side of the matrix converters is based on a    three-phase design. In comparison to a single-phase design, this    allows somewhat lower harmonics on the network side. However, a    staircase voltage that is sufficiently low in harmonics and has a    freely variable number of voltage steps is still impossible.

SUMMARY OF THE INVENTION

An object of an embodiment of the invention is to specifypower-electronic converters and associated control methods which can beimplemented better and can be used universally without the restrictionsmentioned above.

The circuit arrangement should not require any filter capacitors orsnubber capacitors on the AC voltage side. It should be possible tosmooth the undesirable power pulsation at twice the network frequencywithout major complexity in terms of energy stores, so that this powerpulsation no longer occurs in the medium-frequency transformer and thedownstream loads.

Accordingly, the converter branches of the known matrix converterarrangement, which are fitted with bi-directional switches, are,according to an embodiment of the invention, replaced by any desirednumber of identical two-pole networks, which are connected in series andhave the following characteristics:

-   1) The two-pole networks each have at least one switching state (I)    in which their terminal voltage assumes positive values irrespective    of the magnitude and polarity of the terminal current.-   2) The two-pole networks each have at least one switching state (II)    in which their terminal voltage assumes negative values irrespective    of the magnitude and polarity of the terminal current.-   3) The two-pole networks have at least one internal energy store,    preferably a capacitor.

When they are controlled appropriately, two-pole networks with thecharacteristics stated above, allow applied terminal voltages to bepreset both on the network side and on the medium-frequency side, aswill be explained in the following text.

They do not require any external energy stores for their operation, suchas filter capacitors or snubbing capacitors.

With regard to the complexity in terms of semiconductors and energystores, the advantage of the arrangement according to an embodiment ofthe invention is that all of the elements are used uniformly. This isparticularly true if the entire required semiconductor area and theentire capacitor energy are considered. A two-system configuration (15kV and 25 kV) is entirely possible without any switching operations inthe power section. The relative additional complexity—in comparison to apure 15 kV design—is less than in the case of known circuits.

Further advantages include:

-   The problems are restricted to physically small two-pole networks    with relatively low voltages.-   The strictly modular implementation of the power-electronic    converter formed from any desired number of identical two-pole    networks allows free, spatial arrangement of the components.-   The converter allows staircase voltages with a low harmonic content    to be produced both on the network side and on the medium-frequency    side.-   The number and operating voltage of the traction motors may be    chosen independently of the number and operating voltage of the    power-electronic two-pole networks.-   The amplitude and frequency of the medium-frequency that is produced    may be stabilized purely by control techniques, and independently of    the network frequency and network voltage, in order to provide a    simple feed for auxiliary systems.-   Different network frequencies and voltages can be coped with with    little complexity, without any additional switching devices in the    power-electronic converter, the energy stores, the medium-frequency    transformers or the traction motors.-   In the event of failure of one or more of the two-pole networks, the    operating capability can still be ensured without any additional    switching devices (redundancy).

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages, features and details of the invention will becomeevident from the description of illustrated embodiments givenhereinbelow and the accompanying drawings, which are given by way ofillustration only and thus are not limitative of the present invention,wherein:

FIG. 1 shows the basic circuit of the direct converter according to anembodiment of the invention, in each case having four two-pole networksin each converter branch;

FIG. 2 shows a first variant of a single two-pole network in oneconverter branch;

FIG. 3 shows a second variant of a single two-pole network in oneconverter branch;

FIG. 4 shows a third variant of a single two-pole network in oneconverter branch;

FIG. 5 shows a basic circuit for the direct converter according to anembodiment of the invention as a three-phase embodiment;

FIG. 6 shows a known power supply, without a transformer, withhigh-voltage-insulated traction motors;

FIG. 7 shows a known power supply having two or more medium-frequencytransformers;

FIG. 8 shows the basic circuit of a known matrix converter;

FIG. 9 shows a converter branch in the matrix converter as shown in FIG.6, with two thyristors which can be switched off;

FIG. 10 shows a converter branch in the matrix converter as shown inFIG. 6 with two IGBT transistors, and

FIG. 11 shows a converter branch in the matrix converter as shown inFIG. 6 with two IGBT transistors and diodes connected back-to-back inparallel.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a basic circuit for the matrix converter according to anembodiment of the invention. The network-side voltage U_(N) is appliedbetween the circuit points N₁ and N₂. Four converter branches 20 arearranged in the form of a bridge, on whose bridge diagonal amedium-frequency transformer is located. The converter branches 20 eachinclude four two-pole networks 10, which have the characteristicsdescribed above.

FIGS. 2, 3 and 4 show possible advantageous implementations of thetwo-pole networks 10.

FIG. 2 is also known as a full-bridge circuit of a U converter or as aso-called four-quadrant controller, with the difference that it is usedhere as a two-pole network 10. The bridge circuit is formed from fourIGBT transistors 1, 2; 3, 4; 5, 6; 7, 8. The connections on the DC sideare connected to a capacitor 90.

An embodiment of the two-pole network 10 as shown in FIG. 2 additionallyhas the following characteristics:

-   Switching states III exist, in which the terminal voltage U_(x) of a    two-pole network 10 assumes the value zero irrespective of the    magnitude and polarity of the terminal current.-   A switching state IV exists, in which a two-pole network 10 emits no    energy, but can absorb energy, irrespective of the terminal current    direction.

Another possible embodiment of the two-pole network 10, as shown in FIG.3, also has these additionally advantageous switching states. This is achain circuit, that is to say two branches between the terminals x₁, x₂with IGBTs connected in alternate polarity directions and capacitorswhich connect the junction points of in each case two IGBTs in eachbranch, in the illustrated example in each case three IGBTs 1, 2; 130,140; 3, 4 and 7, 8; 110, 120; 5, 6 in each branch and two capacitors 91,92. The circuit shown in FIG. 3, additionally has two further voltagestages, with a relatively small number of switches.

The chain circuit can be extended by further identical elements, as isshown in FIG. 4, for two branches each having four IGBTs 1, 2; 130, 140;170, 180; 3, 4 and 7, 8; 110, 120; 150, 160; 5, 6 in each branch andthree capacitors 91, 92, 93, thus further increasing the number ofpossible voltage stages.

In general, the state III (in the same way as the switching states I andII) will be used as an additional, discrete voltage stage in the controlmethods described in the following text. The switching state IV can beused in the event of interference and interruptions in operation. Theterminal voltage U_(x) is in this state limited in the same way as byovervoltage protection, which is highly advantageous.

FIG. 5 shows an example illustrating that the direct converter accordingto an embodiment of the invention may also be in the form of athree-phase configuration.

The control method will be explained with reference to FIG. 1. Inaddition, in order to simplify the explanation, it is first of allassumed that the capacitor voltages U_(c) of all the two-pole networks10 are at the same initial voltage value U_(c)=U₀.

Each of the converter branches 20 in FIG. 1 can preset a terminalvoltage U_(x)U ₁ =n ₁ *U ₀U ₂ =n ₂ *U ₀U ₃ =n ₃ *U ₀U ₄ =n ₄ *U ₀

The number of possible, different, discrete values of the terminalvoltage U_(x) depends on the number of switching states of the two-polenetworks 10 and the number of series-connected two-pole networks 10 ineach converter branch 20. For the sake of simplicity, the followingexplanation is based on the two-pole network 10 shown in FIG. 2 and anumber of series switches of x=4. Each FIG. n₁, n₂, n₃, n₄ can thenassume the value range{−4,−3,−2,−1,0,+1,+2,+3,+4}

The network-side voltage U_(N), that is to say the potential differencebetween the circuit points N₁ and N₂, can thus be preset in discrete“staircase steps” with the step height of the initial voltage value U₀between−8U ₀ ≦U _(N)≦+8U ₀such that:U _(N) =U ₁ +U ₂ =U ₃ +U ₄

The medium-frequency voltage U_(M), that is to say the potentialdifference between the primary connections M₁ and M₂ of themedium-frequency transformer, can likewise be preset in discrete“staircase steps” with the step height of the initial voltage value U₀between−8U ₀ ≦U _(M)+8U ₀such thatU _(M) =U ₂ −U ₃ =U ₄ −U ₁

The frequency, phase angle and amplitude of the network-side voltageU_(N) and of the medium-frequency voltage U_(M) can be preset completelyindependently of one another, provided that the desired maximum valuesÛ_(N) and Û_(M) in total do not exceed twice the maximum possiblevoltage in one converter branch 20, that is to say:Û _(N) +Û≦2U _(1max)In the present example:U _(1max)=4*U ₀(and in the same way U_(2max), U_(3max), U_(4max))

In general, it is of interest for the network-side voltage U_(N) and themedium-frequency voltage U_(M) to approach the predetermined nominalvalues, which vary with time, as well as possible by in each caseswitching onwards by one staircase step at any desired times. This canbe done for the network-side voltage U_(N) and for the medium-frequencyvoltage U_(M) completely independently of one another.

In order to raise the network-side voltage U_(N) by one staircase step,n₁ and n₄ must be increased by one or, alternatively n₂ and n₃ must beincreased by one. The medium-frequency voltage U_(M) is not affected bythis.

In order to increase the medium-frequency voltage U_(M) by one staircasestep, n₂ must be increased by one, and n₁ must be reduced by one or,alternatively, n₃ must be reduced by one and n₄ must be increased byone.

All of these switching operations each require a switching state changeby in each case one two-pole network 10 in in each case one of twoconverter branches 20, that is to say a total of two switching statechanges. Since two or more two-pole networks 10 are connected in seriesin each converter branch 20, there are in principle degrees of freedomin the choice of the two-pole network 10 to be switched in the relevantconverter branch 20. These degrees of freedom can advantageously be usedfor the following purposes:

-   In order to reduce the required switching frequency for the two-pole    networks 10.-   In order to allow switching operations for the network-side voltage    U_(N) and the medium-frequency voltage U_(M) to occur at random    times without having to take any account of the restriction by the    minimum switching times of the semiconductors.-   In order to balance the individual capacitor voltages U_(c) in the    individual two-pole networks 10 in each converter branch 20.    The latter requires only one measurement of the capacitor voltages    U_(c). Corresponding methods are, in principle, known.

It should also be mentioned that, in principle, it is also possible toswitch both the network-side voltage U_(N) and the medium-frequencyvoltage U_(M) at the same time but likewise with only two switchingstate changes. This can be used to further reduce the mean switchingfrequency of the semiconductors, although this may also result in minorrestrictions to the waveform of the network-side voltage U_(N) or of themedium-frequency voltage U_(M).

An arrangement as shown in FIG. 1 can advantageously be precharged usinga very low voltage auxiliary voltage source. The precharging processtherefore need not be carried out via switches and resistors on thehigh-voltage side. This auxiliary voltage need only reach the order ofmagnitude of a capacitor voltage U_(c) when the two-pole networks 10 areswitched on successively during the charging process.

The charging process via the medium-frequency transformer can alsoadvantageously be used by means of an existing auxiliary systemconverter, when this energy is available from a battery. The batteryvoltage may be considerably lower than the DC voltage for operation ofthe auxiliary system converter. The lack of switches on the high-voltageside and the charging and testing capabilities before the main switch onthe high-voltage side is inserted are advantageous.

The capability of an arrangement as shown in FIG. 1 to operate can bemaintained even in the event of failures in the area of the powerelectronics and control. This is achieved most easily by the two-polenetworks 10 having the characteristic of producing a short circuitbetween their terminals x₁ and x₂ in the event of any failures. This isgenerally ensured in the case of semiconductor components with pressurecontacts. However, for components with contact wires, it is additionallypossible to arrange so-called transient suppressor diodes between theterminals x₁ and x₂, whose contacts (in this arrangement) need withstandonly currents in the same order of magnitude as the operating currents.

The breakdown voltage of the transient suppressor diodes must bedesigned to be greater than the terminal voltages U_(x) which occurbetween the terminals x₁ and x₂ during normal operation without anydisturbances. If higher terminal voltages U_(x) then occur in the eventof a malfunction (redundancy situation)—as a result of the two-polenetworks 10 being connected in series—these voltages are limited in theshort term by the diode or lead to the diode being permanentlyshort-circuited (impedance tending to zero). Both situations arepermissible or desirable in order to make it possible to continue tooperate the overall system. There is therefore no need for anyadditional switching arrangements.

Exemplary embodiments being thus described, it will be obvious that thesame may be varied in many ways. Such variations are not to be regardedas a departure from the spirit and scope of the present invention, andall such modifications as would be obvious to one skilled in the art areintended to be included within the scope of the following claims.

1. A power supply, comprising: a direct converter, wherein each phasebranch includes a number of identical two-pole networks connected inseries, the direct converter being in the form of a bridge circuit whosebridge arms are the phase branches, wherein the bridge circuit is atleast one of a two-phase and a three-phase circuit, and wherein thetwo-pole networks each have: at least one switching state in which theirterminal voltage assumes positive values irrespective of the magnitudeand polarity of the terminal current; at least one switching state inwhich their terminal voltage assumes negative values irrespective of themagnitude and polarity of the terminal current; and at least oneinternal energy store, and wherein at least one medium-frequencytransformer is connected on a bridge diagonal of the bridge circuit. 2.The power supply as claimed in claim 1, wherein the energy store is acapacitor.
 3. The power supply as claimed in claim 1, wherein thetwo-pole network is a bridge circuit with four electronic switches,whose bridge diagonal is connected to a capacitor.
 4. The power supplyas claimed in claim 1, wherein the two-pole network is a chain circuitwith electronic switches in two parallel branches and capacitors whichconnect the junction points of in each case two electronic switches ineach branch.
 5. The power supply as claimed in claim 4, wherein theelectronic switches are IGBTs connected in alternate polaritydirections.
 6. The power supply as claimed in claim 1, wherein transientsuppressor diodes are connected in parallel with the two-pole networks.